<!DOCTYPE html>
<html class="reftest-wait">
<title>
Opacity transition and animation overridden by !important rule creates a
stacking context
</title>
<style>
span {
  height: 100px;
  width: 100px;
  position: fixed;
  background: green;
  top: 50px;
}
@keyframes Opacity0 {
  from, to { opacity: 0; }
}
#test {
  width: 100px; height: 100px;
  background: blue;
  opacity: 0 ! important;
  transition: opacity 100s steps(1, start);
  animation: Opacity0 100s;
}
</style>
<span></span>
<div id="test"></div>
<script>
window.addEventListener("load", () => {
  var target = document.getElementById("test");
  getComputedStyle(target).opacity;

  // Change the opacity style to 100%.
  target.style.setProperty("opacity", "1", "important");
  // Important style is changed but there is a CSS animation,
  // so transition won't be visible and the CSS animation is overridden by
  // the !important rule. As a result opacity style here should be 100%
  // specified by the important rule, but we should create a stacking
  // context for it because there are animations.

  requestAnimationFrame(() => {
    document.documentElement.classList.remove("reftest-wait");
  });
});
</script>
